Intestinal obstruction

Small versus large bowel obstruction

• Why the caecum perforates in distal large bowel obstruction (for example a sigmoid carcinoma with a 13 cm caecum): with a competent ileocaecal valve, gas cannot escape back into the small bowel, creating a closed loop. By Laplace's law (wall tension = pressure × radius), the widest part, the caecum, has the highest wall tension at any pressure, and its thin wall becomes ischaemic and splits. An incompetent valve lets the small bowel dilate and decompresses the caecum.
• Strangulation: constant pain, fever, tachycardia, localised tenderness or peritonism, raised lactate and white count, acidosis: operate urgently.

Causes

• Intussusception: in infants, lymphoid hyperplasia after viral infection (red-currant jelly stool, sausage mass, target sign); in older children and adults there is usually a lead point: Meckel diverticulum, polyp, lymphoma. Peutz–Jeghers syndrome (dark brown freckles on lips and buccal mucosa, hamartomatous small bowel polyps) causes recurrent small bowel intussusception and raises cancer risk.
• Richter hernia: only part of the circumference of the bowel wall (antimesenteric border) is trapped, usually in a femoral hernia. The lumen stays open, so there is no vomiting or distension and bowel sounds may be normal, but the trapped knuckle can strangulate and perforate silently, presenting as a tender groin lump with sepsis.
• At surgery for a strangulated femoral hernia, incising the lacunar ligament can cut an abnormal obturator artery (corona mortis), and the femoral vein lies just lateral to the neck.
• Pseudo-obstruction (Ogilvie): colonic dilatation without a block in elderly patients after orthopaedic surgery, with sepsis, low potassium, magnesium or calcium, opioids; correct causes, neostigmine or colonoscopic decompression. Paralytic ileus: gas throughout the gut, absent bowel sounds.

Investigation and management

• Bloods (dehydration, hypokalaemia, lactate, amylase), blood gas, erect chest X-ray, abdominal X-ray; contrast CT is the investigation of choice (level, cause, strangulation). Gastrografin follow-through in adhesive obstruction is diagnostic and therapeutic.
• Drip and suck: nasogastric decompression, IV fluids with potassium, catheter and fluid balance, analgesia.
• Adhesional obstruction without strangulation: conservative for 48–72 hours. Operate for strangulation, peritonitis, obstructed hernias, closed loops, a virgin abdomen and failure of conservative care.
• Sigmoid volvulus: flexible sigmoidoscopic decompression with a flatus tube if no ischaemia, then elective sigmoid colectomy; Hartmann procedure if gangrenous. Caecal volvulus: right hemicolectomy.
• Malignant obstruction: resection (Hartmann or anastomosis with defunctioning stoma), colonic stent as a bridge or palliation.
• Bowel viability at laparotomy: colour, peristalsis, mesenteric pulsation, bleeding from cut edges; warm packs and re-inspect.

Peritonitis

• Primary: no gut breach: spontaneous bacterial peritonitis in cirrhotic ascites (ascitic neutrophils over 250/µL; cefotaxime plus albumin), pneumococcal peritonitis in nephrotic children, peritoneal dialysis, TB.
• Secondary: perforation (ulcer, appendix, diverticulum, typhoid, cancer, trauma), anastomotic leak, ischaemia, bile, pancreatitis. Secondary bacterial peritonitis is polymicrobial: *E. coli* is the commonest aerobe and *Bacteroides fragilis* the commonest anaerobe.
• Signs: patient lies still, pain with movement or coughing, guarding, board-like rigidity, absent bowel sounds, tachycardia, shock. The elderly and those on steroids may show little.
• How neutrophils reach the peritoneum: margination as flow slows, rolling on selectins, firm adhesion by integrins binding ICAM-1 and VCAM-1 (upregulated by TNF and IL-1), then transmigration between endothelial cells (PECAM-1) and chemotaxis along C5a, IL-8 and bacterial peptides.
• Erect chest X-ray shows free gas under the diaphragm in about 70–80% of perforations (sit upright 10–15 minutes); CT is more sensitive. Gas persists for days after laparotomy.
• Localisation: the greater omentum wraps inflamed organs and walls off pus. In children under about 5 the omentum is short and thin, so a perforated appendix spreads pus throughout the abdomen; small children also present late with vague histories.
• Management: resuscitation, IV antibiotics against Gram-negatives and anaerobes, NG tube, catheter, source control (closure, resection, washout), percutaneous drainage of localised collections, ICU.
• Complications: subphrenic and pelvic abscess (swinging fever; pelvic: mucus diarrhoea, boggy rectal wall), adhesions, sepsis.

• Perforated duodenal ulcer: sudden severe epigastric pain becoming generalised; NSAIDs, steroids, *H. pylori*; omental (Graham) patch, washout, PPI and eradication therapy.
• Typhoid perforation: third week, terminal ileum, antimesenteric border; repair or ileostomy; high mortality.

Acute appendicitis

• The commonest surgical emergency; peak in the teens and twenties.
• Luminal obstruction (faecolith in adults, lymphoid hyperplasia in children, carcinoid, worms) raises pressure, causing ischaemia, bacterial invasion, gangrene and perforation within 24–36 hours.
• Why pain starts at the umbilicus: early distension stimulates visceral afferents that travel with sympathetic fibres (lesser splanchnic nerve) to spinal segment T10, so pain is felt vaguely in the T10 dermatome around the umbilicus. When inflammation reaches the parietal peritoneum (somatic nerves), pain localises to the right iliac fossa. Anorexia and vomiting follow the pain.
• Lymph drains to ileocolic and superior mesenteric nodes.

• Positions: retrocaecal commonest (about 65–75%; fewer anterior signs), pelvic about 20–30%.
• Alvarado score (out of 10): migration, anorexia, nausea or vomiting, RIF tenderness (2), rebound, fever, leucocytosis (2), left shift; 7 or more is likely appendicitis.
• Tests: white count, CRP, urinalysis (mild pyuria possible), pregnancy test in every woman of childbearing age; ultrasound in children and young or pregnant women (non-compressible appendix over 6 mm); CT in adults when unsure; MRI in pregnancy.
• Differentials: mesenteric adenitis, ectopic pregnancy, ovarian torsion or rupture, PID, ureteric colic, UTI, Meckel diverticulitis, Crohn ileitis, *Yersinia*, testicular torsion, right lower lobe pneumonia, DKA, typhoid.
• Treatment: laparoscopic appendicectomy with antibiotics; antibiotics alone are possible in selected uncomplicated adult cases (about 30% recur).
• Appendix mass (days 3–5): conservative (Ochsner–Sherren); colonoscopy or CT later in older patients to exclude caecal cancer. Appendix abscess: percutaneous drainage.
• Pregnancy: the appendix rises with the uterus, so pain is higher; operate, as perforation raises fetal loss.
• Appendiceal neuroendocrine (carcinoid) tumour: commonest appendix tumour, at the tip, usually incidental; under 2 cm needs appendicectomy alone; over 2 cm or at the base needs right hemicolectomy. A ruptured mucinous tumour causes pseudomyxoma peritonei.

Mesenteric ischaemia

• Acute: SMA embolus (AF), thrombosis, non-occlusive low flow, venous thrombosis; pain out of proportion to the signs, high lactate; CT angiography; revascularise, resect dead bowel, second look.
• Ischaemic colitis: watershed areas at the splenic flexure (Griffith point) and rectosigmoid (Sudeck point); bloody diarrhoea, thumbprinting; usually conservative.

Remember Small bowel vomits early, large bowel distends early, and a competent ileocaecal valve turns the caecum into a bursting closed loop.

Exam trap A Richter hernia can strangulate without obstruction: no vomiting, normal bowel sounds, a tender groin lump.

Exam trap An appendix mass is managed conservatively; an abscess is drained; pain out of proportion suggests mesenteric ischaemia.

How FCPS asks about this

• Caecal perforation in distal obstruction: closed loop with a competent valve; Laplace law makes the widest part fail.
• Umbilical pain in early appendicitis: visceral afferents to T10; RIF pain when parietal peritoneum is involved.
• Obturator and psoas signs: pelvic appendix on obturator internus; retrocaecal appendix on psoas.
• Generalised peritonitis in a 3-year-old: short, thin omentum cannot wall off the perforation.
• Ulcer complications: posterior duodenal ulcers bleed from the gastroduodenal artery; anterior ones perforate.
• Obstruction causes: adhesions, hernias (Richter in femoral hernia), ileocaecal TB, Ascaris in children, Peutz–Jeghers intussusception.
• Peritonitis organisms: *E. coli* and *Bacteroides fragilis*; neutrophils roll on selectins and adhere through integrins.